The present invention relates to a positioning apparatus that uses an electric wave from a positioning satellite, a control method of the positioning apparatus, a control program for the positioning apparatus, and a computer-readable recording medium having the control program for a positioning apparatus recorded therein.
A positioning system that measures the current position of a GPS receiver by using, for example, a GPS (global positioning system) serving as a satellite navigation system has been put to practical use.
The GPS receiver receives a C/A (clear and acquisition or coarse and access) code, which is one of the pseudo random noise codes (hereinafter, referred to as ‘PN code’) carried on an electric wave (hereinafter, referred to as ‘satellite wave’) from a GPS satellite, on the basis of a navigation message (including approximate satellite orbit information (almanac) and precise satellite orbit information (ephemeris)) indicating an orbit of the GPS satellite and the like. The C/A code is a code serving as a basis for positioning.
The GPS receiver determines from which GPS satellite the C/A code has been transmitted and then calculates a distance (pseudo range) between the GPS satellite and the GPS receiver on the basis of transmission time and receiving time of the C/A code. In addition, the GPS receiver is configured to measure the position of the GPS receiver on the basis of pseudo ranges with respect to three or more GPS satellites and the position of each GPS satellite on a satellite orbit (for example, refer to JP-A-10-339772).
The GPS receiver performs code synchronization between the received C/A code and a replica C/A code that the GPS receiver has, thereby calculating a phase (hereinafter, referred to as ‘code phase’) indicating a maximum correlation value. The GPS receiver calculates the pseudo range by using the code phase.
Since the C/A code is carried on the satellite wave, it is necessary to perform synchronization (hereinafter, referred to as ‘frequency synchronization’) between a carrier frequency of the received satellite wave and a frequency inside the GPS receiver in addition to the code synchronization in order to accurately perform the code synchronization.
In the case when the strength of a satellite wave is large and accordingly it is possible to output a correlation result that can be used for positioning every short period of time, for example, every 1 millisecond (ms), the frequency synchronization can be performed by using a PLL (phase locked loop) that serves to modify a frequency on the basis of the correlation result.
However, when the strength of a satellite wave is weak, the frequency synchronization using the PLL cannot be performed, and thus the code synchronization cannot also be performed.
For this reason, there has been proposed a technique in which a carrier frequency is detected by using a correlation result calculated by using a receiving channel for holding synchronization and the frequency synchronization is held (for example, refer to JP-A-2003-255036).
However, when the strength of a satellite wave is very weak, it is necessary to secure a long integration time (time until a correlation result is obtained after integration starts, the time being a part of time obtained by summing up coherent time and incoherent time) for C/A codes, for example, 1 second (s) or more.
Here, as described in the above technique, when the correlation result is fed back to a subsequent correlation process, a large amount time gap occurs, which makes frequency traceability worse.
Thus, a problem occurs in that precision of the code synchronization is reduced and positioning precision is reduced or the positioning cannot be performed.